GPU: make std::is_pointer address-space aware on Metal

The shim's partial specialization on a bare T* does match a pointer type written
out in full, but not one deduced from an argument, which carries its address
space. RDHUtils uses std::is_pointer to keep its pointer overloads apart from
its reference ones, so the reference template was instantiated for a pointer and
dereferenced it as a struct.

metal::is_pointer is address-space aware, so the Metal branch forwards to it.

Author: ktf

GPU/Common/GPUCommonTypeTraits.h

@@ -114,7 +114,13 @@ struct is_pointer_t<T*> {
 };
 template <class T>
 struct is_pointer {
+#ifdef __METAL__
+  // A bare T* partial specialization does not match a pointer type deduced from
+  // an argument, which carries its address space; metal::is_pointer does.
+  enum { value = metal::is_pointer<T>::value };
+#else
   enum { value = is_pointer_t<typename std::remove_cv<T>::type>::value };
+#endif
 };
 
 template <class T>